Lasers are sometimes operated as wavelength-sweeping devices in remote sensing and optical coherence tomography applications, as well as to test telecommunications components, among other applications. Discontinuities in the wavelength sweep or operation of the wavelength outside of a single mode can significantly affect the application that the laser is being used in. For example, the shape of a molecular gas absorption feature may be distorted by a discontinuity—a forward or backward jump in wavelength—or operation of the wavelength outside of a single mode in the wavelength sweep of the laser. In another example, wavelength discontinuities or operation of the wavelength outside of a single mode can reduce the signal-to-noise of an OCT measurement of tissue. Thus, it is desirable to eliminate from swept-wavelength lasers wavelength discontinuities, wavelength non-linearity, and operation of the laser outside of a single mode.
Attempts in the prior art to maintain single mode operation and to control the wavelength versus time sweep profile are numerous, but unsatisfactory. Although it may be possible to carefully maintain single mode operation and control the sweep profile at a point in time, the passage of time or changes in, for example, temperature or humidity will create discontinuities, non-linearities, and cause operation outside of a single mode. For example, mechanically-tuned external cavity lasers operate in near continuous single-mode using an external cavity mechanism coupled with a gain medium. In a typical single mode tunable laser, there are two key elements; a method for changing the wavelength, and another for altering the cavity length to optimize side mode suppression and to maintain single mode operation. In an analogous tunable laser, known as Littman-Metcalf configuration, there is a specific mechanical configuration which constrains the change in wavelength to happen coincident with a commensurate change in cavity length, thus maintaining single mode operation. In these mechanical systems, there is a mechanical construction that constrains the mechanical “path” that is traversed to one in which the wavelength increase is linear, and the path length difference is simultaneously changed in concert with the wavelength increase to maintain single mode operation with good single mode suppression ratio (SMSR). One aspect of the present invention may be thought of as the electrical “path” equivalent of the mechanical “path” in an external cavity laser using the Littman-Metcalf configuration, for example. Operation of the mechanical laser is maintained through accurate, tightly-toleranced components and precision alignment of the cavity, or using elements such as piezoelectric transducers that adjust the cavity length in real-time. Other laser configurations use an intra-cavity element. Over time, however, the alignment of the laser degrades or the components wear, which may cause changes in the sweep profile versus time and operation outside of a single mode. As the ambient temperature, humidity, or pressure change, the alignment can degrade, which can also cause changes in the sweep profile versus time and operation outside of a single mode. Vibrations external or internal to the laser may also misalign the cavity, which again may cause changes in the sweep profile versus time and operation outside of a single mode.
Even in lasers with stable cavities, it is difficult to create wavelength sweeps without wavelength discontinuities. Monolithically-constructed semiconductor lasers are a class of single-mode laser for producing swept wavelengths. Monolithic semiconductor lasers include several sections or segments in the semiconductor, which serve as adjustable cavity mirrors, laser gain, cavity phase and (optionally) external amplification. Examples are Vertical Cavity Surface Emitting Lasers (VCSELs), VCSELs with Micro-electromechanical systems (MEMS) tuning structures, Vernier-tuned Distributed Bragg Reflector (VT-DBR) lasers, Super-Structure Grating Distributed Bragg Reflector (SSGDBR) lasers and similar devices. Because these lasers are monolithic with no moving parts (excepting the MEMs devices), their cavities are extremely stable and can operate in single-longitudinal mode with narrow linewidth and long coherence length. Tunable semiconductor lasers of this class require multiple laser current signals to tune the wavelength, presenting a challenge to creating wavelength sweeps without wavelength discontinuities.
There is a need for a system and method for determining and controlling an electromagnetic radiation source to generate a continuous wavelength sweep that maintains an optimized single mode operation.